Very high frequency antenna for motor vehicles

ABSTRACT

An omnidirectional type antenna for very high frequency receivers in motor vehicles. The antenna utilizes a single conductive element coupled between a very high frequency receiver located below the front windshield and a conductive connection with the body of the motor vehicle at a point immediately above the front windshield and at the center thereof. A series coupling capacitor provides coupling of the conductive element to the very high frequency receiver. The ground connection for the very high frequency receiver is provided by a conductive connection from the receiver to a point on the body of the motor vehicle below the front windshield. The conductive element extends from the point at the center of and above the front windshield vertically downward through or adjacent to the windshield to the very high frequency receiver or extends vertically downward through the corner post between the front windshield and the adjacent side door.

United States Patent [72] Inventors Wilbur F.DuBois;

Conrad 0. Gardner, both of Seattle, Wash. [21] AppLNo. 824,041 [22] Filed May 13,1969 [45] Patented Nov.23,197l [73] Assignee The Boeing Company Seattle,Wash.

[54] VERY HIGH FREQUENCY ANTENNA FOR MOTOR VEHICLES 1 Claim, 3 Drawing Figs.

[52] U.S.CI 343/712 [51] 1nt.C1 H01q 1/32 [50] FieldofSearch 343/705, 708,711,712,713,720,767

[56] References Cited UNITED STATES PATENTS 1,591,601 7/1926 Albro 343/720 2,481,978 9/1949 Clough.... 343/767 3,210,766 10/1965 Parker 343/712 FOREIGN PATENTS 576.856 3/1958 Italy 343/713 1,237,187 6/1960 France ABSTRACT: An omnidirectional type antenna for very high frequency receivers in motor vehicles. The antenna utilizes a single conductive element coupled between a very high frequency receiver located below the front windshield and a conductive connection with the body of the motor vehicle at a point immediately above the front windshield and at the center thereof. A series coupling capacitor provides coupling of the conductive element to the very high frequency receiver. The ground connection for the very high frequency receiver is provided by a conductive connection from the receiver to a point on the body of the motor vehicle below the front windshield. The conductive element extends from the point at the center of and above the front windshield vertically downward through or adjacent to the windshield to the very high frequency receiver or extends vertically downward through the corner post between the front windshield and the adjacent side door.

PAIENTEDnnv 23 IS?! TO ammo amour/v5 Elf/VENT /2 K kmm MUM Y E 0 N 0 M B n M0 WMCVI 5 VERY HIGH FREQUENCY ANTENNA FOR MOTOR VEHICLES This invention relates to very high frequency (V.H.F.) antennas for motor vehicles, and more particularly to a V.H.F. antenna system having omnidirectional characteristics.

While present state of the art V.H.F. receiver design has been able to provide the mobile receiver industry with receivers having excellent noise, sensitivity, and selectivity characteristics, V.H.F. receivers for RM. and EM. stereo reception have not been able to displace the A.M. radio as an entertainment device in motor vehicles. While integrated circuits and other recent technological developments in the fabrication of electronic components and devices and mass fabrication techniques have added to continually lower the cost to the consumer of V.H.F. receivers and make them competitive with AM. receivers, the consumer has been generally unsatisfied with the performance of V.H.F. receivers and has been leaning toward the purchase of stereo tapes and cartridges which provide him with the high quality reproduction he desires when listening to music in his motor vehicle. The same high quality reproduction in music is available through the V.l-l.F. stereo radio receivers only if an antenna having the proper characteristics can be coupled to provide the necessary signal levels and maintain them in fringe areas and during changes of direction of the vehicle with respect to the trans mitting antenna. Various prior art V.H.F. antennas have been developed for mobile installations, but each of the various types represented by the patent literature and those in current use have not provided the several qualities required by the consumer and consequently have been rejected by him as unsatisfactory.

The several characteristics which are required in a satisfactory mobile V.H.F. antenna include the following:

Omnidirectional characteristics. Unless the V.H.F. antenna is omnidirectional, a change of vehicle direction with respect to the transmitting antenna will cause a change in amplitude of the signal'voltage delivered by the mobile receiving antenna at the input of the receiver. Where the field strength pattern of the antenna is other than omnidirectional, a change in direction may cause abrupt loss of signal and consequent reception or other undesirable effects.

b. Satisfactory gain. If an antenna is made omnidirectional at very high frequencies, there is in many instances a consequent loss of gain. While omnidirectional characteristics can be achieved to some degree, for example, by a monopole type antenna, its gain is not appreciable over a reference dipole. While techniques are known in V.H.F. antenna design for increasing the gain of an omnidirectional antenna by vertical stacking of dipoles, such as in the well-known collinear type antenna arrangements, such structures become undesirable for mobile installations because of size, necessity for supporting structure, increased wind resistance, etc., and consequently such techniques for increasing the gain of omnidirectional V.H.F. antennas are generally limited to installations ofa fixed nature.

The several characteristics which are desirable in a satisfactory mobile V.H.F. antenna include the following:

a. The antenna should be aesthetically pleasing to the eye. The ultimate in this regard would be a V.H.F. antenna which is completely concealed or hidden from view.

b. The V.H.F. antenna should be inexpensive. Known types of V.H.F. antennas sometimes materially increase the cost of the entire radio receiver system installation, particularly where there is a labor cost of cutting through the vehicle body and installation outside the factory.

c. The antenna installation should be resistant to shock and vibration damage and not susceptible to damage by vandalism.

It is therefore an object of this invention to provide a V.H.F. antenna which provides the above delineated characteristics both those specified as desirable and those which are required.

lt is yet a further object of this invention to provide for improved performance in a mobile V.H.F. receiving system by providing an antenna in the system which is omnidirectional and responsive to both horizontally and vertically polarized waves and waves having the polarization of the type presently utilized by frequency modulation (F .M.) stations in the broadcast service.

It is still another object of this invention to provide an omnidirectional type antenna arrangement of minimal cost for reception of V.H.F. frequency modulation broadcast signals in the 88-108 megacycles band.

Other objects of this invention will become apparent from the following description when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view of the interior of a motor vehicle illustrative of one embodiment of a V.H.F. antenna system arrangement coupled to a receiver in accordance with the teachings of the invention.

FIG. 2 is a perspective view of the interior of a motor vehicle showing a further embodiment of a V.H.F. antenna system arrangement coupled to a receiver in accordance with the teachings of this invention.

FIG. 3 illustrates in block schematic form details of the receiving system including transmission line coupling to the antenna system arrangements of FIGS. 1 and 2.

in accordance with the invention, a V.H.F. receiver is coupled to different points on the conductive body portion of a motor vehicle to provide both the antenna arrangement and ground connections for the V.H.F. receiver. The location of the antenna arrangement connection to the conductive body portion is in the region above the front windshield of the motor vehicle. The ground connection from the receiver occurs on that portion of the conductive body below the windshield. The particular configurations of antenna arrangements, transmission line arrangements, manner of coupling to the V.H.F. receiver and ground for the V.l-l.F. receiver illustrated provide the desired omnidirectionality and gain characteristics.

Referring now to FIG. 1 of the drawings, reference numeral 10 refers to the lower body portion of an automobile body, and including the frame portion at or below the level of the decorative finish moulding 22 which runs along the bottom edge of the conventional automobile front windshield l4 and all portions of the automobile in conductive contact therewith which do not extend above the level of the decorative finish moulding 22. A decorative strip extends around the window opening of wraparound" Windshields in modern automobiles and having an upper portion 20 running along the upper edge of the front windshield l4 and a lower portion 22 running along the lower edge of the front windshield 14. The side portions 61 of the decorative strip are coextensive with and cover a portion of the sideposts 18 of the motor vehicle. The sideposts 18 separate the lower body portion 10 from the upper body portion or roof portion 16 of the motor vehicle. A conventional automobile includes a rear view mirror 26 mounted on a mounting bracket 24 suspended from the upper body portion or roof portion 16 of the automobile directly behind the center of the upper portion 20 of the decorative strip and also behind the midpoint of the upper edge of the windshield 14. The upper portion 20 of the decorative strip extends along and closely adjacent to the upper edge of the front windshield 20. The upper body or roof portion 16 of the motor vehicle includes all the metallic roof portion forming the top exterior surface of the automobile and all conductive material in conductive contact therewith above the corner posts and top level of the front windshield 14 covered by upper portion 20 of the decorative moulding strip. The screws 29 which hold the mounting bracket 24 suspended from the upper body portion or roof portion 16 make conductive contact with and therefore form part of the roof portion 16. The antenna arrangement installed in accordance with the principles of the present invention in the manner illustrated in HO. 1 is coupled to a receiver 30 capable of V.H.F. reception in the manner shown in more detail in FIG. 3.

While attempts have been made to derive radio frequency signal energy at broadcast band frequencies of 550 to 1600 kilocycles from metallic bodies of automobiles as exemplified,

for example, in U.S. Pat. No. 2,353,l l I, such efforts have been largely unsuccessful for several reasons including the pick up of undesirable signal impulses representing high frequency interference originating in the ignition system of the automobile itself and other surrounding automobiles. Indeed, in the above-referred-to patent and in U.S. Pat. No. 1,943,394, attempts were made to overcome this problem by utilizing particular types of circuit arrangements or modifying the antenna arrangement itself. The trend has been away from the earlier attempted use of antenna arrangements utilizing direct coupling to the metallic body of the automobile at A.M. broadcast band frequencies because of the various deficiencies including noise developed by such antennas. Presently a monopole antenna has been used extensively for reception of signals in the AM. broadcast band and efforts have also been made to avoid coupling to the automobile metal in the design of new antennas as, for example, in U.S. Pat. No. 3,414,902.

Coupling across discontinuities in the conductive bodies of motor vehicles to derive radio frequency signals has been ef fected at broadcast band frequencies as shown in U.S. Pat. No. 2,923,813 where the window openings constitute the discontinuities and also at EM. band frequencies as typified by U.S. Pat. No. 2,575,471 where coupling was made to a long narrow air gap or slot in the body surface such as the trunk slot. Such coupling, however, has resulted in non-omnidirectional types of radiation patterns, and in the former reference U.S. Pat. No. 2,923,813 such deficiency is sought to be overcome by coupling also to a second window on an opposite side of the vehicle (see col. 8, line 62), whereas in the latter U.S. Pat. No. 2,575,47l the cardioid pattern which results in less signal strength from the direction in which the vehicle is facing is accepted (see col. 3, line I 1). While signal voltages have been developed across different discontinuities such as the trunk slot or windows as mentioned above, directional characteristics of such antennas are accentuated which are dependent on their location at the back or side of the vehicle. The antenna arrangements of FIGS. l and 2, however, rely upon development of the V.l-l.F. signal voltage between points on the roof portion 16 and the lower body portion 10. The hereinafter described V.I I.F. receiving apparatus derives its input signal voltage between a point on each of these separated surfaces which are in fact separated by the window discontinuities distributed around the vehicle between these two surfaces. The signal voltage is not developed from the perimeter of or directly across the single discontinuity such as a window or trunk slot. Excellent omnidirectionality characteristics are achieved where the signals are derived between the points shown in FIGS. 1 or 2. In practicing this invention it was found that the gain of the antenna arrangement illustrated in FIG. l exceeded the gain of the installation shown in FIG. 2. While the theoretical explanation of the manner in which the new antenna system functions is quite complex due to the complex nature of the surface area over which the signal voltages are developed, since the gain increased when the contact to the roof portion was located at the center where the dome light is located, it would appear that if the windows are acting as discontinuities and developing signal voltages as has been indicated in prior art theoretical explanations, then better coupling to more discontinuities is being achieved in the arrangement of FIG. I than in the arrangement of FIG. 2.

In the antenna arrangement illustrated in FIG. 1, electroconductive element 12 is coupled from a point 34 on the upper body portion or roof portion 16 to the input terminal 50 (shown in FIG. 3) of the receiver 30 which is capable of receiving and processing very high frequency modulated broadcast signals in the 88-l08 megacycles band and providing audio output to the speaker system 53 (shown in FIG. 3). The signals are developed between point 34 on the upper body portion 16 and another point 37 termed ground connection for the receiving system. This point 37 is in conductive relationship with the lower body portion and establishes the reference point above which the received signal from the antenna arrangement is developed. Point 37 in the arrangement shown in FIG. 1 is the grounded or outer braided portion 36 of shielded cable 38 at the receiver and of electroconductive element 12. The shielded outer braid 36 is brought to the ground connection 37 at the receiver 30 chassis by means of a male type coaxial connector, the center conductor of the coaxial connector being soldered to the electroconductive element 12 and the male connector being positioned in the female connector which is mounted on the receiver chassis, thereby providing a direct electrical connection between the electroconductive element 12 and receiver input terminal 51. The V.I-I.F. receiver 30 is fastened to the lower body portion 10 to provide the ground connection for this receiving system. Point 34 is the screw or fastening point for attaching the dome light 32 to the roof portion 14 of the motor vehicle. While electroconductive element 12 is shown as comprising the inner conductor of shielded cable 38 (which may comprise a low impedance SO-ohm coaxial transmission line) having an outer sheath 36 grounded to the receiver chassis, tests have shown that electroconductive element 12 can comprise an insulated wire. When an insulated wire is utilized, it may be positioned behind the ceiling and corner post interior upholstery of the vehicle. While previously mentioned V.I-I.F. antennas have developed signal voltages directly across a single discontinuity, such as window or trunk lid, the present arrangement can be seen to develop a signal voltage between two large metallic surface areas isolated by discontinuities in the form of windows between them. The contacts made to these large surface areas provide an antenna arrangement having the desired degree of omnidirectionality. The lower or ground side of the antenna arrangement is not adjacent the window frame or discontinuity as heretofore but is positioned well down in the lower body portion at the receiver mounting connection point which is at least 2 or more inches below the front windshield periphery. The upper body contact point 34 was made in the central region of the roof portion 16, specifically to the dome light 32 mounting screw 34 where maximum gain was realized. The signal voltages developed between these two contact points so remotely located in the upper and lower body portions were highly satisfactory. When this particular antenna arrangement was utilized with an RM. receiver having a ratio detector but without limiters, the noise level did not appear as high as might have been expected due to the utilization of large spaced apart metallic surfaces. The gain of this arrangement utilizing insulated wire appeared to exceed the gain of the previously utilized exteriorly mounted monopole antenna while requiring only a single electroconductive element 12 connected and located in the above-described manner.

Referring now to FIG. 2, it can be seen that while the ground return for the V.I-I.F. receiving system remains the receiver 30 chassis coupled to the lower body portion 60 by its mounting arrangement beneath the cowl 60, the signal voltages are developed between a mounting screw 29 for the min ror mounting bracket 24 and the above defined ground return to the lower body portion. The mounting screw 29 provides conductive contact to the upper body portion l6 at its edge adjacent and above the center of the front windshield 14. While this arrangement provides somewhat less gain than the arrangement shown in FIG. I, such location minimizes the need for shielding of the electroconductive element 12 extending from a point on the upper roof portion l6 to the receiver input terminal 51 and also permits a minimum length of electroconductive element 12. Since electroconductive element 12 in this arrangement is comprised of a first section comprising the wire element 40 passing in a generally vertical direction downward, inside or on the surface of the windshield 14, it is not required to be insulated here nor shielded since not running along a low impedance metallic body surface, and the lower section 48 of electroconductive element 12 which is coupled to the end of element 40 is very short in length and not required to be shielded since the distance on to the receiver input terminal is less than 2 feet in most cases, depending upon exact receiver mounting position in the particular type and style motor vehicle. Techniques for placing the wire elements within the windshield 14 which may be of the laminated type are shown in the previously referred to US. Pat. No. 3,4l4,902. The method of coupling lower section 48 through metal foils 46 and 42 to wire element 40 is the same as that shown in FIG. 1 of US. Pat. No. 3,414,902 wherein braid 32 is coupled through corresponding foils 48 and 46 to wire element 40:. In FIG. 2 of this application, lower section 48, which is simply an insulated length of wire, is brought through aperture 59 in the cowl down to the receiver terminal 51. The overall physical length of the electroconductive element 12 made up of wire element 40 and lower insulated wire section 48 is less than wavelength in free space at the lowest frequency of the very high frequency band tuned by receiver 30, the total physical length of electroconductive element 12 including sections 40 and 48 being about 48 inches. A single length of insulated wire about 48 inches long, coupled between mirror bracket 24 and receiver input terminal 51, in tests provided excellent reception in weak signal areas when the receiver 30 used was a Sears, Roebuck and Co. model number 833.62830 all transistor A.M./F.M. l2-volt portable autoradio, and the receiver was tuned to stations in the EM. band. The electroconductive element 12 which may be a single section of wire or length of low impedance (50 ohm) type coaxial cable as in FIG. 1, or may comprise two sections which include a wire element 40 and insulated wire 48 as in FIG. 2 is coupled to the receiver input terminal 51 as shown in FIG. 3. The electroconductive element 12 may be incorporated in all vehicles during the vehicle assembly process previous to the insertion of interior decorational material and coupled in accordance with the teachings of this invention to provide a highly satisfactory V.H.F. antenna arrangement of minimum cost to the industry. A series capacitor 52 may then be coupled between receiver input terminal 51 and the high side terminal 50 of the antenna input coil, the capacitor 52 having a range of 7 picofarads to 40 picofarads. This capacitor 52 is peaked to provide maximum signal in the receiver.

When capacitor 52 was peaked for a midband F .M. station, it was not found necessary with this antenna arrangement to readjust the matching capacitor 52 at either end of the FM. broadcast band to provide adequate signal strength from stations located near the band edges which is indicative of the desirable broadband characteristics of this antenna configuration. Where electroconductive element 12 was about 48 inches long, the capacitance setting of capacitor 52 for maximum signal strength of a midband station in the receiver was found to be about 22 picofarads. The receiver 30 utilized in tests of the type and model number previously identified did not include limiter circuitry and therefore the noise characteristics of test antenna arrangements could be monitored and were found to be much lower than expected. The noise level was found to be higher in configurations tested which utilized an unshielded electroconductive element 12 passing along the comer post 18 as in FIG. 1 instead of down along the windshield as in FIG. 2, which effect was probably due to increased coupling to the metallic body in the latter case. The above-identified receiver is shown as block 30 in FIG. 3, the receiver circuit beyond F.M. R.F. amplifier stage 70 utilizing transistor 70 is well known and as illustrated may be typically of the previously identified type and model number. Speaker leads couple the audio output signals from the receiver to speaker system 53. Capacitor 52 shown serially connected between the receiver input terminal 51 and antenna input coil terminal 50 may be located inside the receiver as illustrated or may be serially connected instead between the receiver input terminal 51 and electroconductive element 12 and located physically outside the receiver 30.

While receiver 30 has been shown herein as a V.H.F. receiver capable of tuning the FM. broadcast band for entertainment, the receiver 30 may be a fixed frequency or tunable V.l-I.F. receiver capable of operation on a frequency or frequencies assigned to and utilized by other services such as, for example, police, fire, marine, television or aeronautical.

The antenna structures herein shown may be utilized in accordance with the techniques described on motor vehicles of various types having an upper body or roof portion 16 and lower body portion 10 separated by discontinuities formed by windows 14 or the like which extend between and separate the large metallic surfaces 16 and 10.

We claim 1. In a motor vehicle having a metallic conductive roof of generally rectangular dimension disposed above the front windshield thereof and separated from a metallic body portion disposed below said front windshield by window discontinuities distributed around the vehicle between said roof portion and said metallic body portion, a very high frequency receiving system comprising in combination:

a very high frequency receiver for said system, said very high frequency receiver being electrically grounded to said metallic body portion;

a very high frequency antenna arrangement for coupling very high frequency signals to said receiver, said highfrequency antenna arrangement comprising, an electroconductive element extending from said conductive roof portion to said receiver, said electroconductive element being coupled to said conductive roof portion at substantially the geometric center of said rectangular dimension for achieving omnidirectionality characteristics, and means for electrically insulating said electroconductive element from said metallic body portion.

t t 4 1 i 

1. In a motor vehicle having a metallic conductive roof of generally rectangular dimension disposed above the front windshield thereof and separated from a metallic body portion disposed below said front windshield by window discontinuities distributed around the vehicle between said roof portion and said metallic body portion, a very high frequency receiving system comprising in combination: a very high frequency receiver for said system, said very high frequency receiver being electrically grounded to said metallic body portion; a very high frequency antenna arrangement for coupling very high frequency signals to said receiver, said high-frequency antenna arrangement comprising, an electroconductive element extending from said conductive roof portion to said receiver, said electroconductive element being coupled to said conductive roof portion at substantially the geometric center of said rectangular dimension for achieving omnidirectionality characteristics, and means for electrically insulating said electroconductive element from said metallic body portion. 